1. Field of the Invention
The present invention relates to an R-T-B system rare earth sintered magnet comprising R (R represents one or more rare earth elements), T (T represents at least one transition metal element essentially containing Fe, or Fe and Co), and B (boron) as main components.
2. Description of the Related Art
Among rare earth sintered magnets, an R-T-B system rare earth sintered magnet has been adopted in various types of electric equipment for the reasons that its magnetic properties are excellent and that its main component Nd is abundant as a source and relatively inexpensive.
However, such an R-T-B system rare earth sintered magnet with excellent magnetic properties also has several technical problems to be solved. Such a technical problem is corrosion resistance. That is, since the R-T-B system rare earth sintered magnet comprises, as main constituent elements, R and Fe, which are easily oxidized, it is poor in corrosion resistance. Thus, the surface of the R-T-B rare earth sintered magnet is generally covered with a corrosion resistant overcoat. As such an overcoat, metal plating or resin is used depending on purposes. However, needless to say, it is desired that the corrosion resistance of the sintered magnet itself be improved to achieve higher corrosion resistance.
Another technical problem of the R-T-B system rare earth sintered magnet is mechanical strength. That is, since the R-T-B system rare earth sintered magnet is produced by the powder metallurgy, its mechanical strength is not necessarily sufficient. Thus, when the sintered magnet is applied to a thin magnet, working is not easy.
Japanese Patent Application Laid-Open No. 8-330121 describes the improvement of the corrosion resistance and mechanical strength of a sintered magnet. It proposes that a carbon-enriched layer having a concentration that is 2 times or more than the mean carbon concentration of a sintered magnet is formed on the surface of the sintered magnet at a thickness between 3 and 300 μm. Japanese Patent Application Laid-Open No. 8-330121 discloses that the carbon enriched on the surface of the sintered magnet forms a carbon-R system compound together with R contained in the sintered magnet, and that this carbon-R system compound enhances the strength of the sintered magnet as well as acting as a corrosion resistant overcoat. In addition, the same publication also discloses that when the thickness of the carbon-enriched layer is less than 3 μm, its effect is not exhibited, and when the thickness is over 300 μm, magnetic properties are significantly decreased. The same above publication also discloses a method of forming a carbon-enriched layer, which comprises immersing a compacted body before being sintered in a butyl alcohol solution, in which 5% by weight of carbon powders are suspended, at a room temperature for a certain period of time.
Higher properties are required for an R-T-B system rare earth sintered magnet. In particular, in response to-the-needs for miniaturization of electronic equipment, a further improvement of mechanical strength is required.